Exim is a mail transfer agent for Unix systems connected to the Internet.
It is a monolithic MTA designed to be a command line compatible drop-in
replacement for Sendmail.
Exim is an excellent mailer for an ISP, as its control and flexibility
are very good and its requeueing and retry algorithms are very powerful.
Exim's configuration syntax is well documented.
WWW: http://www.exim.org/